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I cannot recall this very clearly so I'm sorry if this is all not correct but someone once told me that the word 友達{ともだち} has the plural marker たち "built-in" and therefore you cannot say 友達たち about friends in a situation where you could normally use ～たち.

Is that right? If it is, is there any other way to mark plurality for the word 友達 in situations where ～たち is used for other nouns?

2 Answers
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友達 is kind of an odd case - it's a word in the process of fossilisation. 友 on its own is a valid word, albeit one with a distinctly archaic flavour. -たち was then added to make a collective plural (as Thomas Gross says, not a true 'more than one' plural, but instead a 'group described by this term' plural). Modern speakers, though, would always use 友達 in all cases where 友 would have been used in the past (excepting intentional archaisms), meaning that 友達 now also has a singular meaning - the -たち part has fused onto the end of the word and is no longer considered separable. Most people probably wouldn't even think of this -たち here as the collective plural -たち.

'Built-in' is not a term I would use to describe this, though. It's not that 友達 was created as a single noun with -たち already a part, it's instead that the phrase 「友だち」 was so much more common than 友 on its own that people started hearing 友達 as a single unit rather than 友+たち.

However, it's not completely fossilised for all speakers, as some will still mark ?友達たち as sounding odd. (Others are quite fine with it - it's totally fossilised for them.) How do you go about making it plural, then? It's quite simple - the plural of 友達 is 友達. You don't have to do anything with it for it to sound quite natural. If you -really- feel the need to emphasise that you're going with your friends as a group rather than just one, I suppose you can take your chances and hope your listener is fine with 友達たち, but you can easily get away with not bothering - 友達と一緒に行く is quite valid for both 'go with a friend' and 'go with my friends (in general)' (though not for 'go with (some) friends' or 'go with (a couple of) friends'!), and you can always clear up any confusion later.

First, ～たち is notbuilt-in. The noun 友 can appear on its own. See here for more information. Therefore, the answer to your last question is no.

I want to mention though that ～たち, or suffixes such as ～ら, ～ども, etc. do not mark the plural in the strict sense, but rather an associative. An expression such as 犬たち does not simply mean something like English 'dogs', but it means that multiple animals of the dog-persuasion are present in perception. Compare the next two examples:

(a) 公園では犬たちが遊んでいる。
'Dogs are playing in the park.'
(b) * 犬たちは哺乳類だ。
'Dogs are mammals.'

Example (a) presupposes that the speaker is aware of the dogs in question. They are unified as an association of animals in the perception of the speaker.
That, however, would not be appropriate for (b) because the sentence is more general and makes a statement about a larger association that is not present in perception. Hence in (b), the suffix should be absent.